Well to be fair, its not just decaying dinosaurs...
In fact, it is probably more likely decaying peat moss...
Firt forming a peat bog, which gets buried and compressed to form a coal field which is further metamorphisized into an oil deposit.
All happening over millions and millions of years.
But what if (and there possibly is) in addition to that oil, there was additional oil formed from another nearly completely abiotic independant process?
It could very well be true.